Ultraviolet light polarizers from vinyl monomers polymerized in the presence of polyphenyl chain transfer agents



June 4, 1300 OPTICAL DENSITY H c. HAAS 3,386,979

ULTRAVIOLET LIGHT POLARIZE RS FROM VINYL MONOMERS POLYMERIZED IN THEPRESENCE OF POLY'PHENYL CHAIN TRANSFER AGENTS Filed May 15, 1963 300 400wave length '(m u) INVENTOR.

flab M W ATTORNEYS 3,386,979 ULTRAVIOLET LIGHT POLARIZERS FROM VINYLMONOMERS POLYMERIZED IN THE PRESENCE OF POLYPHENYL CHAIN TRANSFER AGENTSHoward CLHaas, Arlington, Mass., assignor to Polaroid Corporation,Cambridge, Mass, a corporation of Delaware Continuation-impart ofapplication Ser. No. 154,338, Nov. 22, 1961, This application May 15,1963, Ser. No. 280,578

Claims. (Cl. 260-891) This invention relates to light polar-izers and tomethods for making the same.

This application is a continuation-in-part of application Ser. No.154,338, filed Nov. 22, 1961, and now abandoned.

This invention has for its primary objects novel light polarizingdevices and novel methods for making the same.

One object of this invention is to provide novel, colorless, visuallytransparent light polarizers for the ultraviolent region.

Another object is to provide a novel light polarizer com prising apolymer prepared by polymerizing a vinyl monomer in the presence of apolyphenyl or substituted poly-- phenyl.

Still another object is to provide a novel process for the manufactureof light polarizers of the character described wherein the molecules ofsaid polymer are oriented to substantial alignment.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the process in= volving the severalsteps with respect to each of the others, and the product possessing thefeatures, properties and the relation of elements which are exemplifiedin the following detailed disclosure, and the scope of the applicationof which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawing wherein:

FIGURE 1 shows the ultraviolet dichroic behavior of the polymer preparedin accordance with this invention.

In the past, useful light polarizers of the ultraviolet region havecomprised oriented materials containing there in a suitable dichroicmaterial. Examples of such polarizers are disclosed in the copending US.applications of Albert S. Makas, Ser. No. 117,827, filed June 19, 1961,now Patent No. 3,254,561 and Ser. No. 128,675, filed Aug. 2, 1961, nowPatent No. 3,276,316.

It has been found in the present invention that an ultraviolet lightpolarizer can be prepared by molec-ularly orienting a polymeric sheetcomprising a linear polymer prepared by the polymerization of a vinylmonomer in the presence of an aromatic hydrocarbon, e.g., diphenyl,which absorbs incident radiation in the ultraviolet region.

The light polarizers of this invention may be prepared by polymerizing avinyl monomer in the presence of a poly phenyl ultraviolet absorbingdichroic compound, preferably diphenyl. A film is cast from theresulting polymer and molecularly oriented, as by stretching.

It should be understood that the term polyphenyl, as used herein, isintended to include substituted polyphenyls, e.g., alkyl substitutedpolyphenyls.

The light polarizer of the present invention exhibits positive dichroismfor ultraviolet radiation, that is, the absorption axis for the incidentultraviolet light is parallel to the stretch axis of the polymericsheet.

In a preferred embodiment, vinyl acetate is polymerized in the presenceof diphenyl. The resulting polymer is iso lated and the acetate groupsare hydrolyzed. A film is 3,3863? Patented June 4, 1968 cast andstretched, and the polymer is then allowed to harden in the extendedcondition. The dichroic properties of the resulting light polarizer werethen measured on a. Cary spectrodensitometer with a Glam-Foucault prism.

The following nonlimiting examples illustrate the preparation of lightpolarizers within this invention.

EXAMPLE 1 A 5050 mixture by weight of vinyl acetate and di phenyl and0.2% of azobisisobutyronitrile were heated at C., for 4 days in a glassampoule sealed under vacuum. The polymer was isolated by precipitationin hexane and then purified by several precipitations from acetone intohexane to remove any unreacted diphenyl. The acetate groups on thepolymer were then hydrolyzed by refluxing for 20' min. in methanolcontaining sodium methoxide. The resulting polymer was purified byseveral reprecipitations from water into methanol. A film was then caston glass, dried and stretched to five times its original length to a 1mil. thickness. By measurement on a Cary spectrodensitometer with aGlam-Foucault prism, the polarizer exhibited a dichroic ratio of 2.7 at258 m EXAMPLE 2 A vinyl acetate-diphenyl polymer was prepared andhydrolyzed, as in Example 1. The polymer was then di luted on a 1 to 1ratio by weight with pure polyvinyl alcohol. A film was then cast and apolarizer was prepared and measured by the same procedure described inEx" ample 1. The polarizer exhibited a dichroic ratio of 3.14 at 258 myand 2.40 at 242 mp.

EXAMPLE 3 Vinyl chloride and. diphenyl (2 to 1 ratio by weight of themonomers) were heated for two days at 60-70 C., with 0.2% ofazobisisobutyronitrile in a glass lined stain= less steel bomb. Theresulting solid was dissolved in a tetrahydrofuran and then precipitatedby hexane. The polymer was purified by several reprecipitations. A filmwas then cast from tetrahydrofuran, dried and stretched. The re= sultingpolarizer exhibited a dichroic ratio of 1.5 at 255 me. The polarizerthus prepared shows substantially higher dichroism in the ultravioletregion than could be attributed to vinyl chloride alone. I

It had been reported that in the polymerization of vinyl acetate in thepresence of benzene, the benzene acts as a monomer and a linearcopolymer is formed containing recurring phenylene units (Journal of theAmerican Chemical Society, volume 82, p. 4780-5, 1960). Utilizing thisprinciple, a vinyl monomer was polymerized in the presence of apolyphenyl compound in order to prepare a copolymer containing adichroic material as part of the polymeric chain. The polymer whenoriented, as by stretch ing, absorbs incident ultraviolet radiationwhose electric vector is parallel to the stretch axis and transmits,substantially unaffected, incident ultraviolet radiation vibrating at aangle to the stretch axis.

However, the theory which held benzene to be a co= monomer has beenrecently refuted (Monatsh, Chem., 92, p. 1100, 1961 and J. Am. Chem.Soc., 84, p. 4986, 1962). Accordingly, it has also been found that thepolymer prepared for the novel polarizer of the present invention is nota copolymer but rather a vinyl polymer containing polyphenyl residues.Polyphenyls were found to act as chain transfer agents and not as acomonomer as was believed initially.

In order to prove that the vinyl polymer chain transfers with polyphenylrather than copolymerizes, a study of the polymerization systeminvolving vinyl acetate and diphenyl was undertaken.

Pure diphenyl in alcohol solution exhibits a single absorption peak at247 m l with an extinction coefiicient of about 2x10 The ultravioletspectrum of an aqueous solution of PVA containing diphenyl residues(prepared from a polyvinyl acetate polymerized in the presence ofdiphenyl according to the procedure set forth in Example 1) has twoabsorption maxima at about 242 and 251 m Since radical attack, at leastby phenyl radicals, has been shown to occur at all three ring positionsof diphenyl (J. Chem. Soc., 794, 1954), the presence of more than onemaximum is not unexpected. The ultraviolet dichroic behavior of a 1 miloriented film of this polyvinyl alcohol diluted one to one with acommercial polyvinyl alcohol sold under the trade name Gelvatol 2/75, byShawinigan Resins Co., Springfield, Mass, is presented in FIGURE 1wherein d represents the curve obtained when the stretch direction ofthe polarizer is parallel to the e vector of the radiation and arepresents the curve obtained when the stretch direction of thepolarizer is perpendicular to the e vector of the radiation. Dilutionwith normal PVA was desirable for reducing the optical density in the250 m region. The best results obtained yielded dichroic ratios of 2.40and 3.14 respectively for the 242 and 258 m bands. The change fromaqueous solution to PVA film resulted in a slight shift of theabsorption maximum of the lower frequency band. After severalpolymerizations of vinyl acetate in the presence of diphenyl, it wasdefinitely established that diphenyl residues are present in thepolymer, that they can be detected and estimated by ultravioletabsorption spectra, and that several species are present since diphenyland mono or dialkyl substituted biphenyls all show a single absorptionmaximum in this range.

To study the polymerization of vinyl acetate in the presence ofdiphenyl, the polymerizations described in Table I were carried out. Thecatalyst, a,a'-azobiscyclohexanecarbonitrile was prepared according tothe procedure of Thiele and Heuser, Ann, 290, 1 (1896) and Dox, J. Am.Chem. Soc., 47, 1473 (1925).

4 following equation found in J. Am. Chem. Soc., 84, 4780 (1960).

log F,,- -3.24+1.40 log [n] TABLE II [1 acetone, C. T;

by disproportionation, f is the fraction of catalyst radicals initiatingpolymerization, rate constants k k k k and k; are respectively forpropagation, initiation, termination by combination, termination bydisproportionation, chain transfer to monomer and chain transfer tosolvent. The [Cat] /2/M ratio was maintained constant for all values ofS/M (see Table I) so thatthe first term on the right hand side ofEquation 2 remains constant. A chain transfer constant, k /k of 6.4 10-was obtained from the above-mentioned plot.

The polyvinyl acetates employed in the viscosity meas urements wererecovered by evaporation of the acetone and converted to polyvinylalcohol by alkaline alcoholysis, care being taken to recover all thepolymer. Quantitative ultraviolet absorption spectra were then obtainedon aqueous solutions of the five samples. In Table III, the

TABLE I.POLYMERIZATION OF VINYL AE ECIATE IN THE PRESENCE OF DIPHENYL,59.

Vinyl Acetate Diphenyl Catalyst, Polymerlza- Polyvinyl Tube M./l. X10tion Time, Acetate G./25 cc. M./l.- G./25 cc. M./l. min. Yield, g

Polymerizations were carried out at 596 C. in sealed glass tubes whichwere carefully evacuated prior to seal ing The mixtures were made up byplacing the biphenyl in a 25 cc. volumetric flask, adding the catalystand dilutspectral data and the chain transfer constant for diphenyl ascalculated from a copolymerization type treatment in which one of the rsis zero are given.

. mg to volume at 59.6 C. with vinyl acetate monomer. 1+k2[M]gk2[M]Viscosity measurements were made on the polyvinyl MS] [648.] (3) TABLEIII Optical Diphenyl Diphenyl PVA Sample PVA, Density at Residues,Moles] Residues, k lk, 254 my 10 cc. 10 Mole/Mole of X10 PVAc Chains 1Cell pathlength is 1 am. 9 Blank.

acetates at 25 C. with acetone as the solvent and aCannon-Ostwald-Fenske viscometer. In Table II, the intrinsic viscositiesare recorded together with the number With regard to Table III, althoughthere are two maxima in the ultraviolet, they are so close together thatthe assumption of a single maximum at 254 Ill/l. introduces averagedegrees of polymerization calculated from the very little error. Opticaldensity measurements for samples 2 through 5 were corrected forabsorption at 254 m by diphenyl free polyvinyl alcohol (sample 1). Anextinction coeflicient of 20,800 was used to calculate the diphenylcontent. This is a. reasonable value for 2 of the 3 possible monoalkylbiphenyls and for 4 of the 6 dialkyl biphenyls in which both rings aresubstituted and is somewhat high for the other mono or di substitutedpossibilities. Therefore, there might be a tendency to minimize slightlythe amount of biphenyl residues present. The variation in k /k values ofTable III cannot be accounted for but they are all certainly inreasonable agreement with the value of 6.4x 10* obtained from R,measurements. The results imply that diphenyl is behaving as a chaintransfer agent and not as a comonomer.

Although the kinetic data are limited, it has been interpreted in lightof Kices scheme for retarded polymerization, I. Am. Chem. Soc., 76,6274, (1954). The symbolism of Peebles, Clark and Stockmayer, J. Am.Chem. Soc., 84, 4780 (1960), has been used. The interpretation yieldedthe following equation:

a2= Qs Qs/ where M .as ea rd) Q dt /04kt m 4 Qu k2 [c3 EmployingEquation 5 and the data of Table I, values of Q and Q for variousinitiator concentrations were calculated (Table IV) and from a plot ofTABLE IV Cai -X10 QoXl0 QsXlO Q SM- (Q Q vs. Q /M a value for {i of 267was obtained from the slope. With k =2.1 10 min.- for azobiscyclohexanecarbonitrile at 59.6 C., it can be easily shown that k /k =0.3 Unlikethe results obtained by Peebles et al., J. Am. Chem. Soc. 84, 4780(1960), for benzene, this kinetic transfer constant is smaller thanvalues obtained from the 1 and analytical measurements. The result ofthe kinetic data lends credence to the viewpoint that diphenyl is notcopolymerizing with vinyl acetate.

In a recent publication, I. Am. Chem. Soc., 84, 4986 1962), based on theethylene-benzene system, it was concluded that the free radicalcopolymerization of benzene is impossible. It is also stated thatbenzene produces essentially no retardation in the rate ofpolymerization of ethylene and argue in favor of the transfer step asbeing The polymerization of vinyl acetate by azonitrile catalysts shouldresult in a rate equation involving M. Cat The data of Table I readilydemonstrate that diphenyl retards the rate of polymerization of vinylacetate. It is therefore difficult to understand how in the vinylacetate system,

the transfer step can be of the type described by Equation 5 since C Hor being active radicals, should not result in much rate dimi nution. Onthe other hand, the postulate that the growing chain does add todiphenyl to produce a radical which is resonance stabilized, has alonger lifetime, and finally either ejects H- or abstracts H- from vinylacetate, is more in accord with the facts.

( From the foregoing study, it can be reasonably assumed that thepolyphenyl is present in the polymer as the arc matic species since onetransfer mechanism (loss of I-I-) produces that directly andphenylcyclohexadiene types should be readily reoxidized by air duringthe prolonged workup.

The novel process of this invention permits the preparation of a lightpolarizer having a desired absorption band by the selection of aparticular dich'roic polyphenyl compound to be present during thepolymerization of the vinyl monomer. For example, diphenyl produces apolarizer in the 250 m region and higher polyphenyls, e.g., terphenyl,quaterphenyl, produce polarizers with dichroism at corresponding longerwavelengths.

By means of this invention, it is possible to prepare polarizers for awide range of the ultraviolet region.

The quantity of polyphenyl associated with the vinyl polymer is notcritical since the relative amount can be readily controlled by dilutionof the polymer with pure vinyl polymer. The higher the ratio ofpolyphenyl units associated with the polymer, the higher the extinctioncoefiicient, but the thinner the film has to be to provide a polarizerwith reasonable transmittance. Therefore, the preferred procedure wouldbe to prepare a polymer initially with a relatively high ratio ofpolyphenyl to vinyl polymer. From this starting point, a film of anydesired composition and thickness can be obtained by appropriatedilution and stretching. The process of this invention, therefore,permits great flexibility and control in the cornposition of thepolymer.

The novel light polarizers of this invention are colorless, visuallytransparent sheets that absorb incident ultraviolet light, whoseelectric vector is vibrating parallel to the axis of orientation, andtransmits substantially unaffected ultraviolet light whose electricvector is at a angle to the angle of orientation.

It may also be desirable to modify the polarizer of this invention insome manner to improve its mechanical stability, for example, bycross-linking the polymer, as by borating, to render the polymer morewater and heat insensitive. The process of borating a light-polarizingfilm is described in US. Patents Nos. 2,554,850, issued May 29, 1951 and2,445,581, issued July 20, 1948. The polarizers of this invention mayalso be coated or bonded to a transparent support to provide protectionagainst physical damage.

A polarizer of the present invention may also be corn-= bined with apolarizer of the visible spectral regions in order to provide thecombined effects of plane polarized light components in both the visibleand ultraviolet regions. Such a combination visible-ultravioletpolarizer may be prepared by placing a polarizer of this invention. anda visible polarizer in contiguous relationship to each other, or byincorporating a dichroic dye for the visible region into the ultravioletpolarizer of this invention at a 7 suitable point in the preparation ofthe polarizer.

It should be understood that in any modification made in the ultravioletpolarizers of this invention, for example, bonding to a support,combining with .a visible polarizer, etc., care must be taken that nomaterial or treatment is used that will significantly affect theabsorption of radiation or impair the efficiency in the etfective rangeof the polarizer.

Since certain changes may be made in the above products and processeswithout. departing from the scope of the invention here-in involved, itis intended that all matter contained in the above description or shownin the accompanying drawing shall be interpreted as illustrative and notin a limiting sense What is claimed is:

1. An ultraviolet light polarizer comprising a molecularly oriented filmcomprising a polymer prepared by polymerizing a vinyl monomer selectedfrom the group consisting of vinyl chloride and vinyl acetate in thepresence of a free radical catalyst and a polyphenyl chain transferagent comprising less than five phenylene groups.

2 A product as defined in claim 1 wherein said vinyl monomer is vinylacetate and said polymer .is hydrolyzed at some time prior to orienting.

3., A product as defined in claim 1 wherein said vinyl monomer is vinylchloride,

4. A product as defined in claim 1 wherein said polyphenyl chaintransfer agent is diphenyl.

5., The process of forming an ultraviolet light polarizer comprising thesteps of polymerizing a vinyl monomer 8 selected from the groupconsisting of vinyl chloride and vinyl acetate in the presence of a freeradical catalyst and a polyphenyl chain transfer agent comprising lessthan five phenylene groups, casting a. film of the resulting polymer,and orienting said film to substantial molecular alignment.

6, A process as defined in claim 5 wherein said vinyl monomer is vinylacetate and said polymer is hydrolyzed at some time prior to saidorienting.

7. A process as defined in claim 5 wherein said vinyl monomer is vinylchloride.

8, A process as defined in claim 5 wherein said polyphenyl chaintransfer agent is diphenyl= 9. An ultraviolet light polarize-rcomprising a molecul-arly oriented film com-prising a vinyl polymerselected from the group consisting of polyvinyl chloride and polyvinylalcohol containing linear polyphenyl chain terminating groups comprisingless than five phenylene groups.

10. A product as defined in claim 9 wherein said vinyl polymer ispolyvinyl alcohol,

References Cited UNITED STATES PATENTS 2,328,903 9/ 1943 Hey'mann 842,400,877 5/1946 Dreyer 117- 12-4- 3,254,56-2 6/1966 Blout et a1. M88-65 JOSEPH L. SCHOFER, Primary Examiner,

M. L. BERCH, Assistant Examiner UNITED STATES PATENT OFFICE CERTIFXCATEOF CORRECTION Patent 79 Dated June 4, 1968 Inventofls) Howard s It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 7, claim 1, line 20 after "a" insert linear-.

Column 8, claim 5, line 3 after "a" insert t linear--.

Signed and sealed this 2nd day of May 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTISGHALK Attesting Officer Commissionerof Patents

1. AN ULTRAVIOLET LIGHT POLARIZER COMPRISING A MOLECULARLY ORIENTED FILMCOMPRISING A POLYMER PREPARED BY POLYMERIZING A VINYL MONOMER SELECTEDFROM THE GROUP CONSISTING OF VINYL CHLORIDE AND VINYL ACETATE IN THEPRESENCE OF A FREE RADICAL CATALYST AND A POLYPHENYL CHAIN TRANSFERAGENT COMPRISING LESS THAN FIVE PHENYLENE GROUPS.
 9. AN ULTRAVIOLETLIGHT POLARIZER COMPRISING A MOLECULARLY ORIENTED FILM COMPRISING AVINYL POLYMER SELECTED FROM THE GROUP CONSISTING OF POLYVINYL CHLORIDEAND POLYVINYL ALCOHOL CONTAINING LINEAR POLYPHENYL CHAIN TERMINATINGGROUPS COMPRISING LESS THAN FIVE PHENYLENE GROUPS.